1. Technical Field of the Invention
The present invention relates to a laser annealing apparatus for performing an annealing treatment on a workpiece to be processed by irradiating the workpiece with laser light.
2. Description of the Related Art
In recent years, a thin film transistor (TFT) in which a polysilicon film is used for a channel layer has been widely used as a switching element of a semiconductor device and a liquid crystal display. The polysilicon film used for the channel layer of the thin film transistor is generally manufactured using a process called a low-temperature polysilicon. The low-temperature polysilicon is a process of forming an amorphous silicon film on a glass substrate and irradiating the amorphous silicon film with a laser light to produce a polysilicon film.
In such a low-temperature polysilicon, the amorphous silicon film is heated, melted, and recrystallized to produce a polysilicon film using a laser annealing apparatus. However, the low-temperature polysilicon has a problem in that if oxygen is included in the annealing atmosphere, the silicon is oxidized during heating to have an adverse effect on the device characteristics. For this reason, an operation of preventing oxidation of silicon using an inert gas such as nitrogen is performed.
In Patent Document 1, a stage on which a glass substrate is placed is provided in a processing chamber so as to supply nitrogen gas into the processing chamber, thus preventing oxidation of silicon during an annealing treatment.
In Patent Document 2, nitrogen is discharged from a tip end of a swing nozzle that swings up and down so as to create a nitrogen atmosphere only in the vicinity of a laser irradiation portion, thus preventing oxidation of silicon during an annealing treatment.
In Patent Document 3, in a state in which a cylindrical cell of which the top portion is sealed by glass and the bottom portion is open is disposed at a position sufficiently close to a substrate, nitrogen gas is filled into the cell, thus preventing oxidation of silicon during annealing.    [Patent Document 1] Japanese Patent No. 3735394    [Patent Document 2] Japanese Patent No. 3091904    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-87962
When the laser annealing using the supply of nitrogen as described above is performed, due to a difference in the flow rate of the nitrogen gas and variation in the ambient air temperature resulting from the change of seasons or the like, a problem that the process becomes unstable occurs, such as the occurrence of irradiation unevenness.
The present inventors have carried out intensive investigations into the cause of the occurrence of such irradiation unevenness and found that a refraction phenomenon of laser light resulting from fluctuation (variation) in the temperature of the nitrogen gas was the cause. In Japanese Unexamined Patent Application Publication No. H11-176730, it is described in regard to a laser interference length measuring machine that variation in the refractive indices of air has an influence on the precision of an optical sensor, which supports the idea of the present inventors.